In many processes, it proves useful to deposit a layer of controlled thickness of a given substance on the surface of the thread so that, in a subsequent manufacturing step, the thread can be processed more easily.
This is the case for example when it is desired to use the thread as a fibre for reinforcing a plastic. It is then necessary to treat the thread so as to make it adhere perfectly to the matrix of the material in question, by depositing a coupling substance on the surface of the thread so as to make the cooperation between these two components as effective as possible. This type of application is widely used in the tire industry or in the reinforced plastics industry.
In the context of the present description, the term “thread” should be understood in a very general sense, covering a monofilament, a multifilament, a cabled or folded yarn or an equivalent assemblage, whatever the material, textile or metallic, of which the thread is made.
More particularly, the invention relates to treatments in which, during a first step, a thin film of a treatment substance is deposited on the surface of the thread in liquid form. This treatment substance may for example be formed from a solvent and an active solute, or else from a substance capable of curing.
The purpose of the following treatment step is to extract the solvent so as to leave on the surface of the thread only the active substance or to allow the curing reaction to be completed, or else to carry out a combination of these two actions.
Very particular attention must therefore be paid to the precise amount of liquid present on the surface of the thread. It is in fact important to ensure that the liquid film deposited, which generally has a small thickness, is as uniform as possible in order to ensure that the properties of the thread are uniform over its entire length.
For this purpose, the known techniques of wetting or coating consist in making the thread run through a bath containing the treatment liquid that it is desired to deposit. By dipping the thread into the bath, it is impregnated with the liquid solution and then emerges therefrom, going towards the treatment step during which the drying or curing takes place.
However, it has been observed that, depending on the nature of the thread or the liquid to be deposited, the thread could behave as a veritable pump, capable of entraining therewith an amount of liquid much greater than the necessary amount. This pumping effect is associated with parameters such as the viscosity of the liquid, the difference in surface tension between the thread and the treatment substance in liquid form, and the run speed of the thread through the bath.
This is why it is recommended to use one or more sets of scrapers or wipers, placed at the outlet of the dipping bath and capable of extracting the superfluous amount of liquid. However, these means may prove to be limited in so far as they are themselves liable to become blocked owing to the many contacts existing between the thread and the draining means, thereby limiting their capability of fulfilling their function, in particular when, for obvious productivity reasons, it is desired to increase the run speed of the thread. One object of the invention is to provide a solution to this problem. Such a device is described by way of example in the publication DE 43 08 889.